1. Field of the Invention
The invention is related to the field of acoustic velocity well logging of earth formations. More specifically, the invention is related to methods of determining changes in acoustic velocity of the earth formations which may be caused by mechanical and chemical alteration related to drilling a wellbore through the earth formations.
2. Description of the Related Art
Wellbores are drilled through earth formations for, among other reasons, extracting useful materials such as petroleum. Wellbores are usually drilled through the earth formations using a rotary drilling rig. The rig is used to turn a drill bit which cuts away at the earth formations. Commercial drill bits typically cut through the earth formations either by shearing or by compressively fracturing the earth formations. When a wellbore is drilled it is usually filled with a liquid suspension called "drilling mud", which is used to hydraulically clear away cuttings generated by the drill bit and by hydrostatic pressure to prevent pressurized fluids present in the pore spaces of the earth formations from entering the wellbore in an uncontrolled manner.
The drilling mud typically includes water as the continuous liquid phase and includes a number of chemical additives to reduce the effects of water on certain earth formations which may be reactive with water. The action of the drill bit in shearing or compressively fracturing the formations, combined with chemical action of the drilling mud on some earth formations, can change the acoustic velocity of some earth formations in the vicinity of the wellbore.
If the acoustic velocity has been changed in some formations as a result of the drilling process, it can be more difficult, for example, to correlate acoustic velocity well logs made in such formations to surface seismic surveys. Changes in acoustic velocity can also correspond to formations which are highly sensitive to the drilling process and may be mechanically unstable after drilling. Such mechanically unstable formations may be prone to "washing" or "caving" which can increase the risk of the drilling assembly becoming stuck in the wellbore. Such mechanically unstable formations may also be prone to production of formation solids during extraction of the petroleum, which can cause fouling or damage to production equipment.
Formation alteration has been associated with changes in the shear wave velocity in the vicinity of the wellbore wall. Changes in shear velocity in turn result in changes in dispersion properties of dipole acoustic waves propagating through such altered formations. See for example, P. Wu, D. Scheibner and W. Borland, A Case Study of Near-Borehole Shear Velocity Alteration, Paper R, Transactions of the 34th Annual Logging Symposium, Society of Professional Well log Analysts, Houston, Tex., 1993. It has also been observed that a "pressurized" wellbore may, as to some altered earth formations, exhibit less dispersion of dipole acoustic waves than would be the case for a similar composition unaltered formations. See for example, B. K. Sinha and Q. -H. Liu, Flexural Waves in a Pressurized Borehole: A Finite Difference Approach, Expanded Abstracts p. 26-29, 65th Annual International Meeting, Society of Exploration Geophysicists, 1996.
It is an object of the invention to provide a method of signal processing for a dipole acoustic well logging instrument which can be used to identify earth formations which have been altered by the drilling process, by means of determining changes in the dispersion-inducing properties of the earth formations.